1. Field of the Invention
The present invention relates to the isolation of the etiologic agent of avian proventriculitis, its identification as a reovirus and the preparation of a vaccine against the disease.
2. Brief Description of the Prior Art
A clinical syndrome in chickens characterized by stunted growth, poor feathering and an increase in leg weakness has been described (Kouwenhoven, V. et al, Vet. Sci. Com., 2:253-259 (1978)). In affected flocks 5-20% of the chicks exhibit growth retardation by one week of age; an increase in lameness and very poor feather development is apparent by two weeks of age. The most prevalent clinical lesions are an enlargement of the proventriculus with areas of necrosis, hemorrhage, and catarrhal enteritis (Kouwenhoven, V. et al, Avian Pathology, 7:183-187 (1978)). Studies indicate that the syndrome can be reproduced in susceptible one-day-old chicks by direct contact with infected chickens or by the injection of homogenized intestinal tracts from infected chickens (Vertommen, N. et al, Ibid, 9:133-142 (1980)). Biochemical examination of plasma from these chicks indicate a low plasma carotenoid level and an increase in alkaline phosphatase activity.
A number of cases with similar clinical lesions have been seen in all of the major broiler growing areas of the southeastern United States. The problem is highly farm-related and is especially severe on large farms and on farms where brooding and growing are conducted simultaneously. The first clinical signs occur when the chicks are about two weeks of age. The chicks appear chilled and do not consume as much feed as expected; the growth rate is retarded with undigested feed passed in the droppings. Many of the droppings are covered with an orange exudate, frequently leading to a misdiagnosis of coccidiosis. At about three weeks of age, paleness of the shank and very poor feathering are commonly seen. The clinical manifestations of this problem and the lesion patterns are extremely variable, making diagnosis of the syndrome difficult.
Post mortem lesions, like the clinical signs, present a confusing picture for the diagnostician. The lesions are not consistent from bird to bird; however, the lesions seen most consistently include an enlarged proventriculus or an engorgement of the glands in the proventriculus. In several instances, there will be a necrosis of the proventricular glands and a small atonic gizzard. Hyperemia and hemorrhages are frequently seen in the duodenal area, and the lower portion of the GI tract will be ballooned and very friable. Perosis and femoral head necrosis are found in many of the chickens exhibiting the aforementioned lesions, and encephalomalacia is also frequently found.
The disease has been given several different names: malabsorption syndrome, pale bird syndrome, brittle bone disease, helicopter disease, and proventriculitis. The latter ("proventriculitis") will be used throughout the present specification and claims.
In order to further understand the nature and scope of the present invention, it is also worth describing the nature of the so-called "reoviruses". The term "reovirus" (Respiratory Enteric Orphan Virus) refers to a group of RNA viruses that infect both the respiratory and intestinal tracts, usually without producing disease (See for example, Davis, B. D. et al, "Microbiology. Including Immunology and Molecular Genetics," 3rd edition, (1980) Harper & Rowe Publishers, Hagerstown, pp. 1206-1212). They include species that infect humans, birds, dogs and monkeys. They appear to be ubiquitous in nature, since specific viral inhibitors (presumably antibodies) have been found in the serum of all mammals tested except the whale. Humans and many other species, (including cattle, mice and monkeys) are naturally susceptible to reoviruses.
Reoviruses have frequently been isolated from the feces and respiratory regions of healthy persons, as well as from patients with a variety of clinical illnesses, particularly minor upper respiratory and gastrointestinal disease. The relation of these viruses to disease is not clear. In fact, Davis et al, supra, at page 1212, state:
The reovirus infections do not seem to be of great clinical importance, and further studies are needed to define their pathogenic potential. Both the respiratory and the gastrointestinal tracts may well be sources for their spread . . . . Antibodies are also frequently found in various wild and domestic animals, but it is not known whether the animals serve as reservoirs for human infections. PA0 Since the meager data assign a limited pathogenicity to these viruses, specific immunization procedures are not warranted.
This statement, coming in 1980, reflects the belief in the art that reoviruses are not involved in any serious diseases or that, at best, significant research would have to be carried out in order to define their pathogenic potential.
A need, therefore, continues to exist for the identification of the etiologic agent of avian proventriculitis, as well as its isolation and possible preparation of a vaccine against the disease.